1. Field of the Invention
The present invention relates to an apparatus for automatically measuring a blood pressure of a living subject according to so-called oscillometric method.
2. Related Art Statement
The oscillometric blood-pressure measuring method is carried out as follows: A pressing pressure of an inflatable cuff which is wound around, e.g., an upper arm of a living subject, that is, a cuff pressure is slowly changed, and a blood pressure of the subject is determined based on the change of an oscillatory component occurring to the cuff during the changing of the cuff pressure, that is, respective amplitudes of respective heartbeat-synchronous pulses of a cuff pulse wave with respect to the cuff pressure. An automatic blood-pressure measuring apparatus which employs the oscillometric method can measure a blood pressure at a front neck, a finger, or an inferior limb of a living subject where it is difficult to measure a blood pressure according to a microphone-using method. In addition, the microphone-using method requires a microphone to be accurately positioned on an artery of a subject, whereas the oscillometric method just needs a cuff to be wound around a portion of a subject. Thus, the oscillometric method can be easily carried out by both skilled and non-skilled operators. Moreover, the oscillometric method can be carried out at a noisy place, or on a child or an in-shock patient who produces small Korotkoff sounds only. Thus, the oscillometric-type automatic blood-pressure measuring devices have come into wide use.
However, according to the oscillometric method, a blood pressure is determined based on the change of respective amplitudes of respective pulses of the cuff pulse wave occurring to the cuff during a certain measurement-time duration, on the assumption that the blood pressure of the subject does not change during that time duration. If the blood pressure of the subject largely changes during the time duration, the envelope of the respective amplitudes largely deforms, which leads to determining an inaccurate blood pressure or even failing to determine a blood pressure.
It is therefore an object of the present invention to provide an apparatus which automatically measures a blood pressure of a living subject with a high accuracy.
The above object has been achieved by the present invention. According to a first feature of the present invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff when a pressure in the cuff is changed; a pressure-pulse-wave detecting device which includes a pressure-pulse-wave sensor that is adapted to be pressed against an artery of the subject and which detects, through the pressure-pulse-wave sensor, a pressure pulse wave that is produced by the artery when the pressure of the cuff is changed and that includes a plurality of heartbeat-synchronous pulses; a correction-factor determining means for determining, for each of the heartbeat-synchronous pulses of the pressure pulse wave, a correction factor to correct an amplitude of the each heartbeat-synchronous pulse to a predetermined value; a corrected-cuff-pulse-wave-amplitude determining means for multiplying, by the correction factor determined by the correction-factor determining means for the each of the heartbeat-synchronous pulses of the pressure pulse wave, an amplitude of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each heartbeat-synchronous pulse of the pressure pulse wave, and thereby determining a corrected amplitude of the one heartbeat-synchronous pulse of the cuff pulse wave; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective heartbeat-synchronous pulses of the cuff pulse wave, with respect to the pressure of the cuff.
According to this feature, the correction-factor determining means determines, for each of the heartbeat-synchronous pulses of the pressure pulse wave detected by the pressure-pulse-wave detecting device, a correction factor to correct an amplitude of the each heartbeat-synchronous pulse to a predetermined value, and the corrected-cuff-pulse-wave-amplitude determining means multiplies, by the correction factor determined by the correction-factor determining means for the each of the heartbeat-synchronous pulses of the pressure pulse wave, an amplitude of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each heartbeat-synchronous pulse of the pressure pulse wave, and thereby determines a corrected amplitude of the one heartbeat-synchronous pulse of the cuff pulse wave. The respective corrected amplitudes of the heartbeat-synchronous pulses of the cuff pulse wave are freed of the influence of change of cardiac output during the change of the cuff pressure. Since the blood-pressure determining means determines a blood pressure of the subject based on a change of the respective corrected amplitudes of the heartbeat-synchronous pulses of the cuff pulse wave, with respect to the cuff pressure, the determined blood pressure enjoys a high accuracy.
According to a second feature of the present invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff when a pressure in the cuff is changed; a pressure-pulse-wave detecting device which includes a pressure-pulse-wave sensor that is adapted to be pressed against an artery of the subject and which detects, through the pressure-pulse-wave sensor, a pressure pulse wave that is produced by the artery when the pressure of the cuff is changed and that includes a plurality of heartbeat-synchronous pulses; an average-pressure determining means for determining an average pressure of respective reference pressures of the heartbeat-synchronous pulses of the pressure pulse wave that are detected by the pressure-pulse-wave detecting device when the pressure of the cuff is changed; a corrected-cuff-pressure determining means for subtracting, from a pressure of the cuff at a time of detection of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to each of the heartbeat-synchronous pulses of the pressure pulse wave, a pressure difference obtained by subtracting, from the reference pressure of the each heartbeat-synchronous pulse of the pressure pulse wave, the average pressure determined by the average-pressure determining means, and thereby determining a corrected pressure of the cuff, and a blood-pressure determining means for determining a blood pressure of the subject based on a change of respective amplitudes of the respective heartbeat-synchronous pulses of the cuff pulse wave, with respect to the respective corrected pressures of the cuff.
According to this feature, the average-pressure determining means determines the average pressure of the respective reference pressures of the heartbeat-synchronous pulses of the pressure pulse wave that are detected by the pressure-pulse-wave detecting device when the pressure of the cuff is changed, and the corrected-cuff-pressure determining means subtracts, from the pressure of the cuff at the time of detection of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to each of the heartbeat-synchronous pulses of the pressure pulse wave, the pressure difference obtained by subtracting, from the reference pressure of the each heartbeat-synchronous pulse of the pressure pulse wave, the average pressure determined by the average-pressure determining means, and thereby determines a corrected pressure of the cuff. The respective corrected pressures of the cuff indicate respective cuff pressures which would be detected at respective times of detection of the respective amplitudes of the pulses of the cuff pulse wave, in a state in which there would be no influence of change of blood pressure during the change of the cuff pressure. Since the blood-pressure determining means determines a blood pressure of the subject based on a change of respective amplitudes of the heartbeat-synchronous pulses of the cuff pulse wave, with respect to the respective corrected cuff pressures determined by the corrected-cuff-pressure determining means, the determined blood pressure enjoys a high accuracy.
According to a third feature of the present invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject, a cuff pulse wave including a plurality of heartbeat-synchronous pulses occurring to the cuff when a pressure in the cuff is changed; a pressure-pulse-wave detecting device which includes a pressure-pulse-wave sensor that is adapted to be pressed against an artery of the subject and which detects, through the pressure-pulse-wave sensor, a pressure pulse wave that is produced by the artery when the pressure of the cuff is changed and that includes a plurality of heartbeat-synchronous pulses; a correction-factor determining means for determining, for each of the heartbeat-synchronous pulses of the pressure pulse wave, a correction factor to correct an amplitude of the each heartbeat-synchronous pulse to a predetermined value; a corrected-cuff-pulse-wave-amplitude determining means for multiplying, by the correction factor determined by the correction-factor determining means for the each of the heartbeat-synchronous pulses of the pressure pulse wave, an amplitude of one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each heartbeat-synchronous pulse of the pressure pulse wave; an average-pressure determining means for determining an average pressure of respective reference pressures of the heartbeat-synchronous pulses of the pressure pulse wave that are detected by the pressure-pulse-wave detecting device when the pressure of the cuff is changed; a corrected-cuff-pressure determining means for subtracting, from a pressure of the cuff at a time of detection of the one of the heartbeat-synchronous pulses of the cuff pulse wave that corresponds to the each of the heartbeat-synchronous pulses of the pressure pulse wave, a pressure difference obtained by subtracting, from the reference pressure of the each heartbeat-synchronous pulse of the pressure pulse wave, the average pressure determined by the average-pressure determining means; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of the respective corrected amplitudes of the respective heartbeat-synchronous pulses of the cuff pulse wave, with respect to the respective corrected pressures of the cuff.
The third feature includes the above-described first and second features. That is, the blood-pressure determining means determines a blood pressure of the subject based on a change of the corrected amplitudes of the pulses of the cuff pulse wave, with respect to the corrected pressures of the cuff. Since the corrected amplitudes of the pulses of the cuff pulse wave are freed of the influence of change of cardiac output during the change of the cuff pressure and the corrected pressures of the cuff are freed of the influence of change of blood pressure during the same time duration, the determined blood pressure enjoys a high accuracy.